Create custom property for sequencer strip in python
You're going to have to provide a code sample that can be used to illustrate/trigger the problem you are having.  I wrote the following bit of code and it worked exactly as I expected:

import bpy
scn = bpy.context.scene
strip = scn.sequence_editor.active_strip
strip['anchovy'] = 3.3
x = [ s for s in scn.sequence_editor.sequences_all if s.get('anchovy') is not None ]
print(x)
print( [ s.get('anchovy') for s in scn.sequence_editor.sequences_all ] )


As you can see, I was able to select a clip based on the property (list comprehension x).